The present invention relates to the control of display devices made of a matrix of display units where the taking of a dynamic or quiescent mode by the display units will cause an image to form on the display device.
Practical display devices employing arrays of light-emitting diodes, dots that flip, and other units capable of displaying dynamic and quiescent modes have been in use for many years. Display panels comprising a large number of such units in a rectangular array and capable of simultaneously displaying many symbols in a dot matrix form are commercially available. A typical dot matrix for displaying a selected symbol consists of an array of 35 dots arranged in a 7.times.5 array.
Previous display systems, however, have had the disadvantage of requiring relatively large amounts of current flow to form the images upon the displays. In particular, U.S. Pat. Nos. 3,638,215, 3,651,511 and 4,024,531, show the display signs which are activated column by column. Thus, when a display requires changing a whole column, all of the rows will have current flowing through them at the same time. In fact, some displays, such as the one shown in U.S. Pat. No. 3,750,138, call for changing the display a character at a time with even greater resultant energy use. Although U.S. Pat. No. 4,072,937 does disclose a method for writing or erasing a single cell, it is done through use of currents sent through all of the units in that column and row.
This problem becomes particularly acute with respect to signs used on moving vehicles such as buses which must carry their own power supply. Buses have been limited in the past to carrying signs consisting of endless tapes each of which carries a number of messages. This type of system has a number of different disadvantages. For instance, only a limited number of messages may be used and if it is desired to add a new message or change a message, it is necessary to change the entire tape. Although numerous types of electronic visual display systems have been proposed for a moving vehicle, none have gained acceptance.